1. Field of the Inventions
The present invention relates to a method for improving detectability of a data frame transmitting background noise information including a silence descriptor identifier and background noise parameters in a communication system in which the information to be transmitted is formed into data frames, said data frames are subjected to channel coding to form channel-coded frames, said channel-coded frames are interleaved to be transmitted in two or more data transmission frames, and information of two channel-coded frames is transmitted in each data transmission frame. The invention relates also to a communication system providing detectability of a data frame transmitting background noise information including a silence descriptor identifier and background noise parameters, the communication system comprising means for forming data frames of information to be transmitted, means for channel coding said data frames to form channel-coded data frames, means for interleaving said channel-coded data frames for transmission in two or more data transmission frames, wherein information of two channel-coded frames is arranged to be transmitted in each data transmission frame. The invention relates further to a mobile station providing improved detectability of a data frame transmitting background noise information including a silence descriptor identifier and background noise parameters, the mobile station comprising means for forming data frames of information to be transmitted, means for channel coding said data frames to form channel-coded data frames, means for interleaving said channel-coded data frames for transmission in two or more data transmission frames, wherein information of two channel-coded frames is arranged to be transmitted in each data transmission frame. The invention relates still further to a network element providing detectability of a data frame transmitting background noise information including a silence descriptor identifier and background noise parameters, the network element comprising means for forming data frames of information to be transmitted, means) for channel coding said data frames to form channel-coded data frames, means for interleaving said channel-coded data frames for transmission in two or more data transmission frames, wherein information of two channel-coded frames is arranged to be transmitted in each data transmission frame, means for deinterleaving received data transmission frames, means for channel decoding, and means for restoring information from channel-decoded data.
2. Brief Description of Related Developments
In data transmission in the form of data frames, the information to be transmitted is usually divided into data frames of fixed size. In addition to information, the data frames can contain header data and other data required in the transmission of the data frames. The data frames are transmitted from a sender to a receiver via a communication channel, which may comprise e.g. the radio channel or another wireless communication channel. The communication channel is subject to interference, such as ignition interference caused by electric equipment and in wireless data transmission, on the other hand, interference caused by other similar devices, such as radio transmitters. Another significant source of interference particularly in mobile transmitter/receiver devices is the fact that the signal to be received can enter the receiver via several routes of different lengths, wherein distortions are caused in the signal received. Consequently, for eliminating errors of transmission, data frames are usually equipped with error correction data or at least error detection data. One method to add error correction data is the use of so-called convolution codes, i.e. the information to be transmitted is encoded by using a suitable convolution code, wherein the convolution coded information is transmitted to the communication channel. At the receiving stage, a reverse operation is made to distinguish the transmitted information from the received data transmission flow. The error detecting data used is most usually parity checking data which is calculated from the information to be transmitted, or at least part of it. One such known parity checking method is the cyclic redundancy check (CRC). Thus, at the receiving end, the corresponding operation is made on the received information and the parity checking data generated at the receiving end is compared with the received parity checking data. If the data match, the receiving device interprets that the information was received correctly. If the calculated and received parity data do not match, a so-called BFI flag (bad frame indication) is set, to indicate to the receiving device that the received data frame was at least partly incorrect. After this, it is possible to request retransmission or an attempt can be made to interpret the incorrect frame e.g. by extrapolation or interpolation.
In current digital mobile communication systems, also speech is transmitted in the form of data frames. For example in the GSM mobile communication system (Global System for Mobile Communications), in the speech communication channel, most of the digital information generated from the audio signal is protected by error correction coding.
Furthermore, current digital mobile communication systems use a so-called discontinuous transmission, wherein the transmitter can be switched off for the time of pauses in speech. This reduces e.g. the power consumption and increases the usage time of the wireless communication device. Moreover, this discontinuous transmission reduces interference in other simultaneous data transmission connections. It is thus possible to improve the quality of the transmission. In practice, however, the transmission is not cut off for the time of the whole pause, but information is transmitted at intervals on background noise which is generated into audible noise in the receiver, corresponding substantially to the volume and frequency spectrum of noise detected at the transmission end. This generation of background noise is a further facility compared to muting the receiver completely for the time of pauses in speech. This background noise is typically transmitted in so-called silence descriptor frames SID at a lower bit rate than speech.
The frequency of transmitting these silence descriptor frames depends e.g. on the communication system used at the time. For example in the GSM mobile communication system according to prior art, speech coding takes place either at full rate (FR or enhanced full rate EFR) or at half rate (HR). During discontinuous transmission, only every 24th frame is transmitted in an FR channel (every 12th frame in an HR channel). All frames to be transmitted during discontinuous transmission are silence descriptor frames. In future mobile communication systems, it is possible to use e.g. adaptive multirate speech codecs (AMR). In silence descriptor frames of such systems, it is possible to transmit not only background noise but also information on the quality of the backward channel of the channel pair (uplink-downlink) used in the connection. For example, in communication between a mobile station and a network element such as a base transceiver station, the transmitting mobile station measures the quality of its receiving channel, i.e. the downlink of the base transceiver station, and transmits quality information in these silence descriptor frames to the base transceiver station. This quality information must be updated regularly to find out a possible need for change of the channel or the base transceiver station. For example in the AMR system, quality information must be transmitted more often than presently because of the change of the codec; consequently, quality information must also be transmitted as part of silence descriptor frames, which should therefore also be transmitted more often than presently.
In the decoder of the receiving device, such as a base transceiver station, the background noise is generated from the received silence descriptor frames at a different way than speech is generated from received data frames containing speech information. Therefore, the decoder of the receiving device must be capable of distinguishing between silence descriptor frames and speech frames. In systems of prior art, this is achieved in a way that silence descriptor frames include a so-called silence descriptor identifier SID-CW (SID code word). This silence descriptor identifier is prior known to both the transmitter and the receiver. Thus, if the received data frame contains this silence descriptor identifier, the decoder processes the received data frame as a silence descriptor frame.
The probability for the fact that a speech frame is identified incorrectly as a silence descriptor frame is:                                           P            SPioSID                    =                                    ∑                              i                =                0                            Nerr                        ⁢                          xe2x80x83                        ⁢                                          (                                                                                                    N                        SID                                                                                                                        i                                                                      )                            ⁢              0                                      ,                  5                      N            SID                          ,                            (        1        )            
wherein
It is assumed in formula 1 that the probabilities for all possible bit combinations are equal. Moreover, the errors in the transmission channel are assumed to maintain this probability distribution.
The probability that a silence descriptor frame is identified as a speech frame depends e.g. on the conditions in the communication channel. If the communication channel used is of high quality, data transmission is relatively flawless and the error probability rate is low. The number of data transmission errors increases with impaired quality of the communication channel, wherein also error possibilities in the silence descriptor identifier are increased, which increases the probability that the silence descriptor frame is not identified correctly.
The silence descriptor identifier must be sufficiently long to identify the received frames as reliably as possible. If the silence descriptor identifier is too short, the probability that a speech frame is identified as a silence descriptor frame increases. If the silence descriptor identifier or part of it is transmitted in the unprotected part of the data frame, the probability that the silence descriptor identifier contains errors is greater than in a situation where all bits of the silence descriptor identifier are transmitted in the protected part of the data frame.
For example in the GSM mobile communication system, interleaving of data frames is used, i.e., one data frame is not sent in full but it is divided into e.g. four or eight elements. These elements are transmitted in successive bursts, however in a way that one burst contains one element of two different data frames. This interleaving is illustrated in a skeleton diagram in the appended FIGS. 1a and 1b. FIG. 1a shows an example of a full rate speech channel in the GSM system, wherein each data frame to be transmitted is divided into eight elements. In a corresponding manner, FIG. 1b shows a half rate speech channel, wherein the data frames are divided into four elements and transmitted in successive bursts. With this interleaving, an attempt is made to reduce the effect of interferences, which occur typically in bursts in the radio channel, in the reliability of the transmission.
In the system of FIG. 1a, one channel-coded speech frame in the full-rate speech channel consists of 456 bits. This channel-coded speech frame is divided into eight partial blocks consisting of 57 bits so that the first bit (bit 0) is placed in the first partial block, the second bit (bit 1) is placed in the second partial block, the third bit (2) in the third partial block, the eighth bit (7) in the eighth partial block, the ninth bit (8), in turn, in the first partial block, etc. After this, these eight partial blocks are placed in eight bursts so that the bits of the first partial block are placed in the even bits of the first burst, the bits of the second partial block are placed in the even bits of the second burst, the bits of the third partial block are placed in the even bits of the third burst, and the bits of the fourth partial block are placed in the even bits of the fourth burst. In a corresponding manner, the bits of the four next partial blocks are placed in the odd bits of the four next bursts. In this example, each burst consists of 114 bits. The odd bits of the four first bursts comprise the bits of the four last partial blocks of the previous channel-coded frame to be transmitted. In a corresponding manner, the even bits of the four latter bursts comprise the bits of the four first partial blocks of the channel-coded frame to be transmitted next. In this way, one burst comprises, as a rule, bits of two channel-coded frames. One purpose of this arrangement and interleaving is to reduce the effect of interference in the communication channel in several consecutive bits of the same data frame. Thus, errors are distributed in several different data frames, wherein possible bit errors can be better detected and even corrected by error detection and correction methods.
In a corresponding manner, in the half-rate channel of FIG. 1b, one channel-coded speech frame consists of 228 bits and is interleaved in four bursts. Thus, each burst consisting of 114 bits contains bits of two successive speech frames. In practice, this interleaving has the effect that at the moment of cutting off the transmission, the burst of the last element of the data frame to be transmitted contains an extra data frame element to complete the number of bits to be transmitted in a burst (114 bits). However, this extra element is not used at the receiving stage. Correspondingly, in a situation where transmission is turned on again, an extra data frame element is transmitted in the first burst to be used. Also this element is not used at the receiving stage. This interleaving in the GSM system is defined in more detail in the standard GSM 05.03 which also describes channel coding in full-rate and half-rate channels of the GSM system.
In practical communication systems, it is not possible to protect all the bits to be transmitted, wherein some of the data frame bits are unprotected when transmitted. On the other hand, for achieving as reliable a silence descriptor frame as possible, the silence descriptor identifier must be made as long as possible, wherein it is a problem in the systems of prior art that some of the bits of the silence descriptor identifier must be transmitted unprotected, which increases the error rate in the transmission of the silence descriptor identifier to the receiver. For example, in the GSM AMR speech coding method under development, the lowest suggested bit rate can be greater than in the presently used half-rate audio coding in the GSM system, where the bit rate is 5.6 kbit/s. The total rate in the half-rate channel of the GSM system, including also the bits added in channel coding, is 11.4 kbit/s. As a result, in ARM speech coding in the GSM system, there are not necessarily as many protected bits available as in half-rate speech coding of the GSM system presently in use. Furthermore, some of the protected bits are used for the transmission of channel quality data, wherein there are not sufficiently protected bits left for the transmission of the silence descriptor identifier in a sufficiently reliable way. The appended FIG. 2 shows an error rate for the incorrect identification of a speech frame as a silence descriptor frame. In the figure, three silence descriptor identifiers of different lengths are used as examples (44 bits, curve 2A; 89 bits, curve 2B; 118 bits, curve 2C), and the error rate of the half-rate channel of the GSM system of prior art is presented for comparison, wherein the length of the silence descriptor identifier is 79 bits (curve 2D). In a corresponding manner, FIG. 3 illustrates the probability that a silence descriptor frame is identified in the receiver incorrectly as a speech frame. Two silence descriptor identifiers of different lengths are used here: 44 bits (curve 3A) and 89 bits (curve 3B) as well as, for comparison, the error rate of a silence descriptor frame of the GSM 79 bit half-rate channel (curve 3C). The error rates of FIGS. 2 and 3 are calculated as a function of the number of allowed incorrect bits in the silence descriptor identifier. On the basis of FIGS. 2 and 3, it can be noticed that with the silence descriptor identifier lengths of 44 or 89 bits, identification is not as reliable for identifying both the silence descriptor frame and the speech frame correctly as with the 79 bit silence descriptor identifier used in the half-rate channel of the GSM system.
It is an aim of the present invention to achieve a method for more reliable transmission of silence descriptor frames, as well as a communication system. The method of the present invention is characterized in that:
a first silence descriptor frame is formed provided with the silence descriptor identifier,
said first silence descriptor frame is subjected to channel coding to form a channel-coded silence descriptor frame,
said channel-coded silence descriptor frame is transmitted in two or more data transmission frames,
at least one data transmission frame transmitting part of said channel-coded silence descriptor frame is also used to transmit at least the background noise parameter.
The communication system of the present invention is characterized in that the communication system comprises further:
means for forming at least a first silence descriptor frame to be transmitted at a time, said first silence descriptor frame comprising a silence descriptor identifier,
means for channel coding of said first silence descriptor frame, to form a channel-coded silence descriptor frame,
means for transmitting said channel-coded silence descriptor frame in two or more data transmission frames,
means for using at least one data transmission frame, containing part of said channel-coded silence descriptor frame, to transmit at least the background noise parameters.
The mobile station of the present invention is characterized in that the mobile station comprises further:
means for forming at least a first silence descriptor frame (to be transmitted at a time, said first silence descriptor frame comprising a silence descriptor identifier,
means for channel coding of said first silence descriptor frame, to form a channel-coded silence descriptor frame,
means for transmitting said channel-coded silence descriptor frame in two or more data transmission frames, and
means for using at least one data transmission frame, containing part of said channel-coded silence descriptor frame, to transmit at least the background noise parameters.
The network element of the invention is characterized in that the network element comprises further:
means for forming at least a first silence descriptor frame to be transmitted at a time, said first silence descriptor frame comprising a silence descriptor identifier,
means for channel coding of said first silence descriptor frame, to form a channel-coded silence descriptor frame,
means for transmitting said channel-coded silence descriptor frame in two or more data transmission frames, and
means for using at least one data transmission frame, containing part of said channel-coded silence descriptor frame, to transmit at least the background noise parameters.
The invention is based on the idea to utilize interleaving of data frame elements presently used in the transmission of data frames, wherein upon transmission of silence descriptor frames, only the silence descriptor identifier is transmitted in the first data frame, and parameters related to this background noise are transmitted in the next data frame.
The invention gives significant advantages to the methods and communication systems of prior art. The invention makes it possible to distinguish between silence descriptor frames and other data frames in a more reliable way also in data transmission at a lower bit rate than is possible to achieve in methods and communication systems of prior art. As a result, the use of such a communication system is more convenient, because speech and background noise are received more reliably, wherein the intelligibility of speech is improved and also possible disturbing noise occurs less frequently than in communication systems of prior art.